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1. Field of the Invention.
The present invention relates to a small, fast, and energy efficient printed circuit board curer-dryer oven apparatus for liquid photoimaginable solder masks and legend inks. In particular, the present invention has a small compact size resulting from a geometrical component configuration that facilitates the simultaneous application of thermal infrared radiation for curing and drying coupled with pneumatic air for evaporative and conductive cooling of the printed circuit board and for evacuative transport of potentially flammable vapors. This particular employment of curing heat from infrared emitters instead of convection from hot air allows energy efficiency to be obtained from the extremely short preparatory time such emitters require and the elimination of the necessity of keeping the curing chamber at a steady temperature. Known art may be found in U.S. Classes 219, 118, in subclasses 58, 388 respectively as well as in other classes and subclasses.
2. Description of the Known Art
To produce the conductors on printed circuit boards with thin fiberglass expoxy cores that are coated with metallic foil, usually copper, coating with a photoresist or with a similar photosensitive and developable polymer layer is one step in a multiple step process. The desired structure of conductors is exposed on the dried coating, and the coating is either developed at the exposed or with alternate types of photoresist at the unexposed places. In the three step etching process that follows, the photoresist coating is first dissolved away at the non-circuit areas. Then the copper is etched away at the uncovered places, whereas the coated places are protected from the etching effect. Then the remainder of the photoresist coating is dissolved resulting in the in-layer or printed circuit board having the desired structure of conductors.
The printed circuit boards are subsequently processed by applying a solder mask, i.e. a coating applied to a printed circuit board after the circuits are established but prior to component electronic installation. The function of a solder mask is to reduce the ability of the molten solder to adhere to the printed circuit board surface. A solder mask is typically applied by screen printing using artwork which enables the entire board to be covered except for the holes, pads, and contact fingers.
Ideally, after any soldering operation solder adheres only to th solder plated hole, and the component lead in the hole. In reality without solder mask, solder will not only stick to the dielectric substrate (the fiberglass and plastic material which the board is fabricated from), it will also bridge across that substrate and short circuit the conductors. With the application of hard, resistant solder masks, the ability of molten solder to adhere to the board surface is greatly reduced.
There are other reasons for applying solder mask. In actuality, solder mask need only be applied to the solder side (that side which will contact the solder wave). However, it is common for printed circuit users to require solder mask on both the component and solder sides. Solder mask also acts as a barrier which prevents damage to the circuitry due to scratches. Solder mask also prevents short circuits from forming when stray wires or dirt contaminates the surface. Printed circuits that must function in environments of high humidity are better protected from corrosion with solder mask.
The four common types of solder masks are: (1) two part epoxy, (2) one part epoxy, (3) ultraviolet curable and (4) dry film solder mask. The type of solder mask of principal relevance are those with involving epoxy. Previously, the applied solder masks were cured to hardness, heat resistance, and chemical resistance by baking in an oven with hot air circulation. The general objective of the curing process is to remove any volatiles (if present) and to chemically cross link and/or polymerize the solder mask. Curing toughens the solder mask to help ensure that it will maintain its integrity during the chemical, thermal, electrical, and physical exposure the printed circuit board will be subjected to during its service life.
A solder mask coating installation is where the printed circuit boards have a layer or coating applied. Immediately after the solder mask is applied, the board is cured which is a simultaneous hardening and evaporation of volatile compounds. Evaporation and simultaneous hardening has been accomplished in both batch and conveyorised thermal convective ovens and combination thermal radiant infrared (IR) convective ovens. The curing process leaves a liquid mask at a certain hardness that maintains the surface integrity and makes the boards more durable to physical abrasions. The gas, in the curing chamber, generated through evaporation is transported away from the printed circuit board to the outside air. An apparatus for scrubbing the exhaust vapors is usually available to be put in the transport path of the exhaust stream so that any potentially toxic vapors can be captured. Similarly, a clean air filter can be put in the air inlet orifices for removing solid impurities from incoming air.
In order to obtain the desired throughput, printed circuit boards have to be cured in a relatively short time. For that purpose, the curing stations are usually in the form of a conveyorised oven apparatus in which the coated printed circuit board is dried and cured during its transport from an entry opening to an exit opening. The coating has been normally dried in a stream of hot air that is passed over the surface of the board. There are also continuous curers in which the coating is dried and cured by means of convective hot air streams coupled with infrared (IR) radiation. As a rule, circulating hot air drying steps have been preliminary to those involving infrared emitters in order to evaporate off and transport away any flammable solvent present in the coating. Such preliminary drying has been deemed absolutely essential in the case of conventional infrared curers for safety reasons since, without it, the flammable solvent vapors, which would not be evaporated out and off until the infrared emitters were reached, could ignite upon contact with the hot surface of the infrared emitter.
In the past, an important factor determining the duration of the curing of the coating has been the temperature inside the continuous curer. It could not be set to an arbitrarily high level since that would undesirably increase the risk of igniting any remaining flammable solvent vapors. When infrared emitters were used, there was the additional risk that the liquids contained in the coating would evaporate too quickly and form bubbles (boiling bubbles), which could severely impair the evenness and the quality of the coating and could even result in an unusable printed circuit board. There have been, therefore, limits to shortening the throughput times in conventional known continuous curer-driers. The need for consecutive steps in the previous curing and drying techniques have lead to a large dimension in the conveyorised curing and drying path so that each step in the curing-drying process could essentially be completed prior to the commencement of the next step. The need to keep the curing chamber at a given temperature range has operational requirements of either a long warm-up time or that the heating elements be continually on thus requiring a continuous energy input. Energy efficiency could be improved either though shorter warm-up times which would be coupled with not having to maintain a constant temperature in the curing and drying chamber.
The continuous oven described in U.S. Pat. No. 4,978,836 is a continuous infrared oven whose function is to accomplish reflow soldering where surface mounted devices are attached to printed circuit boards. As a solder apparatus, its applicability to the curing-drying process for legend inks and photoimaginable solder masks is only marginal.
Another infrared oven described in U.S. Pat. No. 4,833,301 has a multi-zone temperature environment controlled by the infrared emitters via their control of emitted thermal infrared wavelengths. As this invention pertains to a reflow soldering apparatus, its applicability to the curing-drying process for legend inks and photoimaginable solder masks for printed circuit boards is only marginal.
A continuous oven for board shaped piece material described in U.S. Pat. No. 5,567,237. This device uses three zones of differing wavelength infrared emitters to provide a ramp of incremental temperatures increases coupled with directed air flow parallel to the printed circuit board""s surface and counter to the printed boards travel direction. This arrangement with parallel airflow differs substantially from the curer-dryer oven of the present invention.
Thus, a need exists for an improved small, fast, and energy efficient printed circuit board curer-dryer oven apparatus for liquid photoimaginable solder masks and legend inks that also has auto-loading and unloading capability.
The present invention addresses the need in the art for an improved curer-dryer apparatus. One principal object of the present invention is to provide a conveyorised printed circuit board curer that permits a rapid throughput, that is to say rapid curing of printed circuit boards without at the same time impairing the quality of the coating. In accordance with the present invention, this system utilizes a combination of infrared panel emitters and pneumatic airflow characteristics to control heat accumulation and dispersal as well as to compartmentalize the vapors generated during the curing process.
The present invention efficiently and safely combines directed air flow for drying and heat removal with panel infrared emitters that provide thermal energy for curing printed circuit boards being transported though an area with pneumatic and radiant processes occurring simultaneously. The internal geometric configuration of the apparatus and high velocity input air results in an airflow stream in relation to the panel infrared emitters that ensures any flammable vapors are kept separated from the infrared emitters.
In one exemplary embodiment of the present invention, a conveyorised curing-drying oven uses both convective and radiant heat transport to the printed circuit boards for curing. The infrared panels generally have a high efficiency for energy conversion and generate only a small amount of waste heat. Little or no waste heat results in a curing-drying chamber that requires minimal insulation to protect the operators as well as to provide an acceptable temperature profile in the immediate vicinity of the curing-drying chamber. The curing-drying chamber does not require elevated temperatures or even the maintenance at a given temperature range for curing and drying printed circuit boards. The initialization time, i.e. that time from when the apparatus is switched on until it is ready to begin processing printed circuit boards, is extremely short as the panel infrared emitters are the only operational components that require time to attain the desired operating status. The panel infrared emitters began producing a sufficient radiation flux density in the acceptable wavelengths (for example medium thermal infrared) and attain operational temperature status in less than one minute.
The satisfaction of the requirements with regard to explosion protection is to be maintained to thus avoid ignition of the solvent evaporated during the curing process. This is especially true as it relates to the geometry of panel infrared emitters and transport paths for incoming air and exiting flammable vapors.
In accordance with one exemplary embodiment of the present invention, an oven with a protective shroud and internal frame directs incoming air flow for drying and heat removal through panel infrared emitters that provide thermal energy for curing printed circuit boards being transported though a chamber where the pneumatic and radiant processes are occurring simultaneously. The geometric configuration of the air flow stream in relation to the panel infrared emitters is such that if any flammable vapors are produced, these vapors are kept separated from the infrared emitters.
The conveyorised curing-drying apparatus does not depend substantially upon convective heat transport to the printed circuit boards for curing but does use convective heat transport via directed air to cool the printed circuit boards. The infrared panels generally have a high efficiency for energy conversion and generate only a small amount of waste heat.
The curing-drying chamber does not require elevated temperatures or even the maintenance at a given temperature range for curing and drying printed circuit boards since the initialization time or the time from when the apparatus is switched on until it is ready to begin processing printed circuit boards, is extremely short as the panel infrared emitters are the only operational components that require time to attain the desired operating status. The panel infrared emitters began producing a sufficient radiation flux density in the acceptable wavelengths in less than one minute.
Thus, a principal object of the present invention is to provide an apparatus having highly energy efficient operation as compared to older curing methods and devices.
Another object of the present invention is to provide a highly efficient apparatus with compact dimensions that is inexpensive in design and that require little additional operator instruction for its satisfactory operation.
Yet another object of the present invention to provide a device having the capability of being coupled with various coating or inking installations on such a manner as to further automate circuit board production by allowing the end products of coating or inking to be mechanically input directly into and out of the curing oven without being subjected to manual manipulation.
Another object of the present invention is to facilitate adjustable throughput capacity and productivity to accommodate different requirements of various manufacturing facilities without requiring excessive alteration and modification.
Another object of the present invention is to provide an improved apparatus that produces satisfactory finished components quickly and economically while maintaining a safe operating environment.
A basic object of the present invention is to provide a device using thermal radiation and turbulent airflow to quickly cure and dry printed circuit boards.
Another principal object of the present invention is to provide a device with directed air streams that maximize desirable curing characteristics while minimizing undesirable vapor transportation characteristics to thereby hasten printed circuit board curing and drying while preserving a safe operating environment.
These and other objects and advantages of the present invention, along with features of novelty appurtenant thereto, will appear or become apparent in the course of the descriptive sections.